The invention relates to an industrial grade sodium purification method to reduce it calcium content considerably.
Sodium produced industrially by molten sodium chloride electrolysis generally has a calcium content of 400 to 500 ppm. This content is greater than the solubility limit of calcium in molten sodium at standard operating temperatures in industry, such that there is a risk of a deposit of calcium in the installations carrying the liquid sodium. In addition, for some applications, such as in the nuclear field for example, this purity is insufficient and, for this reason, different purification methods have been proposed and implemented for several decades.
The patent FR1484647, registered in 1965 by Commissariat à l""Energie Atomique, disclosed the use of sodium peroxide to eliminate calcium. The method consists of mixing sodium and peroxide in a reactor at a temperature between 200 and 300xc2x0 C. with vigorous stirring, and eliminating the solid lime particles by filtration. In this way, it is possible to obtain sodium containing less than 10 ppm of calcium.
The applicant""s patent FR 2251627, registered in 1973, perfected the above method to remedy certain drawbacks such as the discontinuous nature of the method, the solid deposits on the walls of the reactor and the very high peroxide consumption.
However, such a method is not economically adapted to the purification of sodium for more standard applications, for which a varying reduction of the calcium content is desired, without being bound by the strict standards of the nuclear industry.
The patent FR 1214176 held by du Pont de Nemours, published in 1960, discloses a sodium purification method consisting of mixing molten sodium at a temperature below 300xc2x0 C. with an inert gas, particularly nitrogen, containing 0.1 to 2%, and preferentially 0.1 to 0.5% oxygen, the gas being recycled after passage. The patent specifies that the addition of water vapour into the inert gas instead of oxygen is ineffective, since it does not help render the calcium insoluble to separate it by filtration, decantation or centrifugation.
The aim of the invention is to offer a sodium purification method by means of calcium oxidation that is simpler and less costly than and at least as effective as the methods of the prior art.
The invention relates to a technical sodium purification method consisting of placing liquid sodium in contact at a temperature greater than or equal to 150xc2x0 C., inside a static mixer, with an inert gas stream with respect to the sodium containing the stoechiometric quantity of water vapour required to oxidise calcium so as to lower its content to the desired content, and then separating the purified sodium from the inert gas, and the hydrogen and lime produced by the reaction.
The invention is based on the observation made by the applicant that, under certain implementation conditions, and contrary to the disclosure in the patent FR 1214176, the use of water vapour as an oxidant of calcium could result in simple and effective sodium purification.
The method is based on the oxidation reaction: Ca+H2Oxe2x86x92CaO+H2. In fact, the reaction probably takes place in two stages via sodium oxide, i.e.: Na+H2Oxe2x86x92NaO+H2 and NaO+Caxe2x86x92Na+CaO. The quantity of water required for a given quantity of sodium is the stoechiometric quantity corresponding to the calcium to be eliminated, i.e. the difference between the initial calcium content and the target content. Since the method is continuous, this implies that the ratio K between the water flow rate and sodium flow rate is kept more or less constant. The operation is carried out in a static mixer favouring satisfactory exchange between the liquid sodium and the gases. The role of the inert gas, which is preferentially nitrogen, is firstly to dilute the water vapour and place it in contact with the sodium, and secondly to evacuate the hydrogen formed by the reaction. The quantity of nitrogen required per gram of water is preferentially between 30 and 40 dm3/g. This quantity is significantly lower than that mentioned in the patent FR 1214176, which recommends a ratio of the dispersion gas volume over the sodium volume between 20 and 200, while the quantity mentioned above, for practically complete calcium purification, results in a ratio of the order of 6. This makes it possible to avoid recycling the gas, which simplifies the installation considerably.
The temperature of the liquid sodium in the mixer must be greater than or equal to 150xc2x0 C. to prevent any risk of fouling of the mixer. It is preferable to avoid excessively high temperatures, for example greater than 250xc2x0 C., both for safety reasons and to prevent corrosion of the installation.
At the mixer outlet, the purified sodium is firstly separated from the gases via a degasser, followed by the lime by filtration.